Tire having sacrificial bridging

ABSTRACT

The present invention provides a pneumatic tire having a tread portion comprising a plurality of axially spaced apart essentially longitudinal grooves separating essentially longitudinal ribs. On at least one of said ribs, transverse grooves or cuts repeat in the circumferential direction to form first and second land portions wherein the first land portions comprise blocks having a circumferential length greater than that of the second land portions. Said second land portion acts as a sacrificial bridge which provides traction improvement and minimizes undesirable surface anomalies during the service life of the tire.

This application is a continuation-in-part of non-provisional U.S.application No. 09/098,395, filed Jun. 17, 1998, now issued as U.S. Pat.No. 6,102,092, and is a continuation of PCT application PCT/US99/13607,filed Jun. 15, 1999, published in the English language as WO99/65814.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a radial pneumatic vehicle tire forwhich tread surface anomalies causing user dissatisfaction arediminished without decrease in tire performance such as wet traction andbraking performance. More specifically, the invention relates to apneumatic tire having a plurality of axially spaced apart essentiallylongitudinal grooves separating essentially longitudinal ribs. On atleast one of said ribs, transverse grooves or cuts repeat in thecircumferential direction to form first and second land portions whereinthe first land portions comprise blocks having a circumferential lengthgreater than that of the second land portions.

2. Description of Related Art

In order to improve the wet traction, wet grip, braking performance andthe like, radial pneumatic tires have treads with longitudinal or zigzaggrooves extending in the circumferential direction, and, for furthertraction improvement, transverse grooves axially connecting thecircumferential grooves to form blocks. To maintain a good level oftraction performance, the transverse grooves or cuts need to be presentthroughout the service life of the tire tread. Unfortunately, to achievethis the tire must have transverse grooves whose depth is substantiallyequal to the depth of the longitudinal grooves. An example of such areference tire 100 is shown in FIGS. 1a and 1 b, respectively, in a fulltire view and a plan view of the tread portion of the tire. In thisexample the tread blocks 20 are circumferentially spaced apart by thesubstantially full depth transverse grooves 30. Tire treads so designedare commonly used on the drive axle of vehicles and have acceptable wettraction performance, but are known to have reduced tread rigidityresulting in the formation of tread surface anomalies such as a“heel-and-toe” or “sawtooth” profile or tread block depression. Theseanomalies result in user dissatisfaction due to either unacceptablevisual appearance of the tire or ride discomfort caused by tread inducedvibrations. Either factor can cause removal of the tire from serviceprior to delivering its full potential tread service to the user.

To achieve some kind of compromise between ace anomalies and tractionperformance, tires have been designed having transverse grooves definingblocks 20 where the transverse grooves 30 have a depth d substantiallyless than the depth h of the longitudinal grooves, an example of whichis tire 200 shown in FIG. 2a. The land portions of the tread bounded byedge 22 of a first block 20 and by edge 21 of a second block 20 arecommonly referred to as “bridges”. For values of d/h near zero, tireswill have poor traction, and for values of d/h approaching unity, tiresmay develop surface anomalies leading to reduced service life of theoriginal tread. An acceptable result can be obtained when the tire treadis designed so that the ratio R₁=d/h of groove depth d to the treaddepth h is such that d/h is between about 0.1:1 to about 0.2:1.Unfortunately, tires experience a loss of tread rubber due to factorssuch as abrasion, fatigue and the like during their service lives. As aresult, tires having tread designs such as shown in FIG. 2a, that iswith shallow transverse grooves, will wear in such a manner that theratio d/h will continually decrease and eventually approach a value ofzero. The disadvantage of such a tire wherein d/h approaches zero is theaforementioned loss of wet grip, braking performance and the like.

Tests under highway use conditions were conducted on tires such as tire200 having a new tire tread depth of approximately 20.5 mm withtransverse grooves approximately 3 mm deep. The evolution of d/h justdescribed is demonstrated by the test results shown in FIG. 2b whichshows the measured tread depth versus circumferential position for asection of the tire. After 54,000 kilometers of service the tread depthhas reached an approximate value of 17 mm everywhere, and the ratio ofd/h is approximately zero. In this case the tires are more often removedfrom service for a perceived loss of traction rather than for the onsetof surface anomalies. In an effort to mitigate this counterperformance,tire designers often add additional siping or employ complex blockgeometry which, instead of improving the situation, may further generatesurface anomalies and/or sensitivity to chipping or tearing. Thus a tiretread design that maintains the optimum value of the ratio d/hthroughout the service life of the tread is needed.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improved radialpneumatic tire which maintains good wet traction performance and is freeof surface anomalies. This object is obtained by a tread portion of thetire having a plurality of longitudinal grooves which form ribs and atleast one of those ribs being transected by narrow transverse groove orcuts which form alternating land portions wherein the first landportions are longer than the second land portions.

According to the notation shown in FIG. 3b for tire 300, the first landportion will hereafter be referred to as block 20 and the second landportion as sacrificial bridge 30. An object of the invention is tomaintain a non-zero value of the ratio R₁=d/h. To accomplish thisobject, the sacrificial bridge must be decoupled from the adjacent treadblocks 20. This decoupling can be achieved by narrow transverse groovesor cuts 40 and 50. Cut 40 is located between the trailing edge 22 of afirst of blocks 20 and a leading edge 31 of the sacrificial bridge 30.Cut 50 is located between the trailing edge 32 of the sacrificial bridge30 and the leading edge 21 of a second of blocks 20. Leading andtrailing edges are defined relative to the rolling direction of the tirewith the leading edge 21 being the first point on block 20 to engage theground during rolling of the tire and the trailing edge 22 being thelast point on block 20 to engage the ground during rolling of the tire.The sacrificial bridge 30 is bounded in its circumferential extent bycuts 40 and 50 and in its lateral extent by circumferential grooves 10.The depth h₁ of the cuts 40 and 50 and the height h₂ of the sacrificialbridge 30 are such that the surface 33 of the sacrificial bridge 30contacts the ground during rolling of the tire under load. An example ofsuch a design is the tire 300 shown in plan view in FIG. 3a.

Since the surface 33 contacts the ground when rolling under load, thesacrificial bridge 30 will be subjected to longitudinal shearing forcesduring the period of ground contact. This shearing force must besufficient to generate a rate of rubber loss (measured in mm/km) fromthe sacrificial bridge such that the ratio d/h is maintained. In orderto solve the problems found in reference tires, the inventor has foundthat an optimum level of shearing force, and thus, rate of rubber losswill be obtained only for certain ranges of the values of R₁=d/h,R₂=h₁,/h, and the ratio of sacrificial bridge length L₂ to block lengthL₁, R₃=L₂/L₁. When these parameters are in their respective optimumranges will the ratio d/h be maintained throughout the service life ofthe tread.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and embodiments will be described with reference to theaccompanying drawings, wherein:

FIG. 1a is a partial perspective view of a reference pneumatic radialtire 100 having full depth transverse grooves.

FIG. 1b is a plan view of the tread portion of the reference pneumaticradial tire 100 shown in FIG. 1a.

FIG. 2a is a plan view of the tread portion of a reference pneumaticradial tire 200 having partial depth transverse grooves.

FIG. 2b is a cross sectional view of the tread portion of the referencepneumatic radial tire 200 as shown in FIG. 2a.

FIG. 2c is a graphical plot of tread depth vs. circumferential positionaround a tire having the tread portion shown in FIG. 2a.

FIG. 3a is a plan view of the tread portion of a pneumatic radial tire300 corresponding to a first embodiment of the invention.

FIG. 3b is a cross-sectional view taken along the midline of the treadportion shown in FIG. 3a wherein the groove edge sipes have been removedfor clarity.

FIG. 3c is a graphical plot of the tread depth vs. circumferentialposition around a tire having the tread portion shown in FIG. 3a. Note:Direction of tire rotation indicated by the uppercase R.

FIG. 4a and FIG. 4b are cross-sectional views of the tread portion of apneumatic radial tire 300 showing possible configurations for inclinedcuts. Note: Direction of tire rotation indicated by the uppercase R.

FIG. 5a is a plan view of the tread portion of a pneumatic radial tire400 corresponding to a third embodiment of the invention.

FIG. 5b is a partial perspective view of the tread portion of apneumatic radial tire corresponding to a third embodiment of theinvention.

FIG. 6a is a cross-sectional view taken along the midline of the treadportion shown in FIG. 5a showing a first cut of zigzag profile.

FIG. 6b is a cross-sectional view taken along the midline of the treadportion shown in FIG. 5a showing a first cut of sinusoidal profile

FIG. 7a is a cross-sectional view taken along the midline of the treadportion shown in FIG. 5a showing a second cut having a curvilinearprofile

FIG. 7b is a cross-sectional view taken along the midline of the treadportion shown in FIG. 5a showing a second cut having a chamfered profile

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

When a block type tire is operated on a vehicle during highway use,surface shearing stresses are developed at the tire-road interface duethe flattening of the tire carcass and belt structure and due tocompression of the tread block elements. Since the tread surface isdisposed at a radially outward position greater than that of the beltstructure, rolling into contact with a flat surface causes a tangentialstress to develop at the tire road interface in an advancing directionduring approximately the first half of contact and in a retardingdirection during approximately the second half of contact. For tireshaving block or block-type tread designs, a second set of stresses isgenerated due to the vertical compressive strain of the tread rubberinduced by the vertical load applied to the inflated tire. This secondset of stresses acts in an advancing sense at the transverse edge of theblock first contacting the ground and in a retarding sense at thetransverse edge of the block last contacting the ground. Thesetransverse edges are referred to respectively as the leading edge 21 andtrailing edge 22 of the block 20 shown in FIG. 3b. The two sets ofstresses act simultaneously on the block surface with a resultant rateof tread rubber loss which can be non-uniform across the block surface.Specifically the rate of tread rubber loss is often a maximum at or nearthe trailing edge 22 of the block. The tread surface profile resultingfrom this non-uniform tread rubber loss is commonly known as a“heel-and-toe” “sawtooth” profile. In a later stage, such a treadsurface profile can result in the rapid depression of some tread blocksrelative to adjacent blocks and may necessitate premature removal of thetire from service.

In the present invention a sacrificial bridge is provided between treadblocks. The presence of the sacrificial bridge minimizes the undesirable“heel-and-toe” or “sawtooth” profile while at the same time maintainingacceptable traction performance. To achieve the above object accordingto the present invention, a radial pneumatic tire has a tread portioncomprising: (a) a plurality of axially spaced apart essentiallycircumferential grooves having a depth h in the tread portion of thetire, and (b) at least one rib formed on the land portion between two ofsaid circumferential grooves, and (c) a plurality of transverse grooveshaving a depth h₁ not exceeding the depth h of said circumferentialgrooves and arranged at circumferential intervals on at least one of theribs, wherein alternating pairs of said transverse grooves define firstland portions having a circumferential length L₁ circumferentiallyadjacent to second land portions having a circumferential length L₂, theratio of the length L₂ of said second land portion to the length L₁ ofsaid first land portion is such that 0.25:1≲L₂/L₁≲0.50:1, and (d) saidsecond land portion is offset radially inward from said first landportion a distance d. By the proper specification of the ratio of thelength of the sacrificial bridge to the length of the tread block andthe depth and width of the cuts at the leading and trailing edges of thesacrificial bridge, the desired effect is maintained throughout theservice life of the tread.

FIGS. 3a and 3 b, 4 a and 4 b show various embodiments of the tireaccording to the present invention. In these embodiments a plurality ofcircumferential grooves 10 are arranged at regular axial intervalsacross the tread portion of the tire. The number and specific axialposition of the circumferential grooves is determined according to theintended application of the tire. Circumferential grooves 10 have adepth h in the radial direction. A plurality of ribs is formed betweenadjacent circumferential grooves. In these embodiments, the ribs aredivided in the transverse direction by a plurality of circumferentiallyspaced cuts 40 and 50. A first land portion, block 20, has a surface atthe most radially outward position of the tread portion of the tire.Block 20 has a length L₁ in the circumferential direction, and a heighth in the radial direction equal to the depth of the circumferentialgrooves 10. A second land portion, sacrificial bridge 30, has a surface33 spaced radially inward from the surface of block 20 by a distance d.Surface 33 of sacrificial bridge 30 has a length L₂ measured in thecircumferential direction. Cuts 40 and 50 have a depths h₁ measuredradially inward from the tread surface and widths w₁ and w₂,respectively, measured in the tire circumferential direction. Cuts 40and 50 are shown in the figures as straight radial cuts having equaldepths h₁ although the invention encompasses cuts 40 and 50 havingunequal depths or differing alternative shapes.

When a tire according to the present invention is mounted on a rim,inflated and loaded according to recommendations of the Tire and RimAssociation, rolling the tire against the ground causes theabove-mentioned sheer stresses to be generated on blocks 20. Whensacrificial bridges 30 are present, compression of the tread rubbercauses the radial walls of cuts 40 and 50 to approach each other so thatsacrificial bridge 30 now acts to buttress the adjacent tread blocks 30against the action of the aforementioned shear forces and therebyimprove the uniformity of tread rubber loss across the surface of theblock. Land portion 33 of sacrificial bridge 30 is also subjected tosimilar stress mechanisms. Due to the presence of cuts 40 and 50, thesacrificial bridge is free to undergo shear deformation and rubber losssuch that the depression d is maintained. If the specific dimensions ofsacrificial bridge 30, are such that the land portion has insufficientresistance to shear deformation, then the rate of rubber loss frombridge surface 33 will be less than the rate of rubber loss form blockportion 20 In this case, depression d disappears after a low number ofservice miles and the ratio d/h approaches zero.

FIG. 3a shows a first embodiment of the tire according to the invention.In this embodiment, blocks 20 are formed in the ribs betweencircumferential grooves with sacrificial bridges 30 being formed betweenthe blocks 20 by radial narrow cuts 40 and 50. Sacrificial bridges 30are located at regular circumferential intervals around thecircumference of the tire. In FIG. 3a, blocks 20 are shown with auniform length L₁. Typically length L₁ is between approximately 1.0% to1.4% of the tire circumference although L₁ may have multiple discretevalues so as to create a sequence of discrete block pitch lengths.Within the teachings of the invention, both the actual values of L₁ andthe sequence of the discrete pitches are typically determined tominimize undesirable tire noise. Sacrificial bridges 30 are shown withstraight edges 31 and 32 having an intersection angle β relative to thetire rolling direction as shown in FIG. 3a. Angle β is preferably in arange of about 60° to about 90°. The invention encompasses edges 31 and32 which may take on alternative zigzag or curvilinear shapes.

EXAMPLE CASE

The invention disclosed herein can be advantageous for all classes ofpneumatic tires where there is a need to improve the compromise betweentraction capabilities and overall service life. In order to demonstratethe improvements possible with the present invention, three differentdesigns according to this first embodiment were prepared on 275/80 R22.5 heavy duty truck radial tires and then were mounted on the driveaxles of 6×4 heavy duty trucks operated under highway serviceconditions. Each design was mounted with a companion set of referencetires. During the course of the test, tread depths and tread surfaceprofiles were measured as well as notations of the appearance of treadsurface anomalies. The specifics of the three designs and the referencetire are shown in Table 1.

From the results of these tests, a tire according this embodiment couldmaintain an acceptable recess and thus d/h for up to 144,000 km (90,000miles). The results shown in Table 1 clearly demonstrate that onlycertain combinations of the design parameters yield this level ofperformance. Between Embodiment 1-1 and Embodiment 1-2, all parametershave been held constant except the initial depression d. Nevertheless,the mileage to d/h˜0 is essentially equivalent at 80,000 km (50,000miles). Embodiment 1-3 exhibits superior performance of 144,000 km(90,000 miles). In this instance, the ratio R₃=L₂/L₁ has a value of0.42:1.

The data show that an effective range for R₃ is 0.25:1≲L₂/L₁≲0.50:1.,and preferably R₃ should have a value greater than about 0.40:1 up toabout 0.50:1. Values of R₃ less than 0.25:1 will yield a sacrificialbridge whose rate of rubber loss will be insufficient to showsubstantial improvement in maintaining the depression d. Converselyvalues of R₃ greater than 0.50:1 mean that the total surface area ofblocks would be insufficient to provide adequate tractive forces orwould produce an accelerated rate of tread rubber loss.

TABLE 1 Example Cases Using Embodiment 1 Tread Design Design Parameters(Ref- Embod- Embod- Embod- See Figure 3b erence) iment 1-1 iment 1-2iment 1-3 Block Length 45 42 42 38 L₁ (mm) Bridge Length 8 12 12 16 L₂(mm) Width of First Cut N/A 0.5 0.5 0.5 w₁ (mm) Width of Second Cut N/A0.5 0.5 0.5 w₂ (mm) Initial Depression 3 2 3 3 d (mm) Tread Depth 20 2020 20 h (mm) Cut Depth N/A 20 20 20 h₁ (mm) Bridge Height N/A 18 17 17h₂ (mm) R₁ = d/h 0.15 0.10 0.15 0.15 R₂ = h₁/h N/A 1.00 1.00 1.00 R₃ =L₂/L₁ 0.18 0.29 0.29 0.42 Distance to d/h ˜ 0 54,000 80,000 80,000144,000 (km)

The ratio R₁=d/h varies in the test cases between 0.10 to 0.15, and aneffective range has been found to be 0.10:1≲R₁≲0.20:1. Preferably R₁ isapproximately 0.15:1. In all examples shown in Table 1 the depth of cuts40 and 50 is equal to the tread depth, h, or, alternatively R₂=1.00:1.However, habitual practice by users of heavy duty truck tires oftenleads to removal of a tire from service with some tread remaining. Thisallows cuts 40 and 50 to be less deep than the tread depth h, but, inall cases, maintenance of acceptable wet traction performance duringactual service requires h₁ to be at least 75% of the tread depth h. Thisleads to a specification of 0.75≲R₂≲1.00 and, preferably that R₂ isapproximately 1.00. The widths of cuts 40 and 50 are the same and equalto 0.5 mm in this embodiment. Widths w₁ and w₂ are effective in therange of about 0.2 mm to about 2 mm. Preferably, widths w₁ and w₂ arebetween about 0.5 mm to about 1 mm. Unfortunately, concentrated stressesat the bottom of narrow cuts 40 and 50 can produce cracking which cancause the early removal of the tire from service. To reduce this stressconcentration, cuts 40 and 50 require a minimum radius at the bottom ofthe groove of about 1 mm. As a means to reduce this stressconcentration, cuts 40 and 50 as well as the groove edge siping areshown in the figures with an enlarged portion at their most radiallyinward extent.

Results from vehicle tests such as those shown in Table 1 and in FIG. 2cindicate that the depression d is maintained well adjacent to theleading edge of block 30 but is considerably diminished adjacent to thetrailing edge. In spite of the improved performance obtained by thepresence of the sacrificial bridge, the tread rubber loss profile ofblock 20 exhibits a tendency for sawtooth shape. A way to obtain adesired improvement of a more uniform height of the block 20 throughoutthe service life of the tread is to incline at least one of the cuts 40or 50 relative to the outward normal from the tread surface.

A second embodiment of the invention is shown in FIGS. 4a and 4 bwherein cuts 40 and 50 may be inclined with respect to the outwardnormal from the tread surface. In these cases the axes of the cuts 40and 50 have inclination angles α₁ and α₂, respectively, relative to theoutward normal from the tread surface. Angle α is positive when thegroove axis is rotated in the direction of tire rotation orcounterclockwise as shown in FIGS. 4a and 4 b. In the first exampleshown in FIG. 4a only cut 40 is inclined in the range −15°≲α₁≲−5° andpreferably α₁ is between about −7° to about −10°. In another example(not shown) only cut 50 is inclined in the range −15°≲α₂≲−5° andpreferably α₂ is between about −7° to about −10°. For the example shownin FIG. 4b, both cuts 40 and 50 are inclined and α₁ and α₂ have negativevalues. In the example of FIG. 4b, inclination of the grooves can beeffective over a range −15°≲α₁ or α₂≲−5° and preferably both α₁ and α₂are between about −7° and about −10°. Inclination of cuts 40 and/or 50causes the tire to be directional, that is, having a preferred directionof rotation. This is shown in FIGS. 4a and 4 b by the uppercase R. It isalso customary for this preferred direction of rotation to be indicatedon the tire by an arrow or an advisory.

In a third embodiment according to the invention, as shown for tire 400in FIGS. 5a and 5 b, increased sliding of the sacrificial bridge isaccomplished by increasing the width of second cut 50 relative to thewidths disclosed for the first embodiment. Another advantage of thisembodiment is that increased sliding of the sacrificial bridge producesan accelerated initial rate of rubber loss from the sacrificial bridge.This permits the sacrificial bridge of the tread to initially have nodepression (d/h=0), a potential advantage in further preventing theearly appearance of tread surface anomalies, while thereafter developinga non-zero value of d/h. Thus both increased service life and tractionon the used tire are obtained. It is not desirable to have an excessiveamount of tread surface area as sacrificial regions for reasons of tireservice life. Thus tire 400 has a narrow first cut 40 of width w₁ closeto the trailing edge of a first block 30 increased width w_(2 of) secondcut 50 close to the leading edge of a second block 30. For tread depthsh of approximately 20 mm, the width w₂ of cut 50 can be approximately 1mm to 10 mm. Increasing the width of cut 50 reduces the rigidity of theentire tread and can adversely affect the rate of tread rubber loss. Asa result it is preferable to maintain width of cut 50 in the range 1mm≲w₂ ≲3 mm. Increasing the width of cut 50 now allows the sacrificialbridge 30 to develop and to maintain its depression longer into the lifeof the tire. Width w₁ of narrow cut 40 remains preferably in the rangeof 0.2 mm to 1 mm.

An important function of the sacrificial bridge is to reduce shearstress generated by compressive loading of the block 30. In the thirdembodiment just described, enlarged second cut 50 reduces thebuttressing effect of the sacrificial bridge which leads to a loss ofrigidity at the trailing edge 22 of block 30. It is advantageous toregain some of this lost rigidity by a form of first cut 40 which“locks” the trailing edge 22 of block 30 to the leading edge 31 ofsacrificial bridge 30. This can be accomplished with a first cut 40having a zigzag as shown in FIG. 6a, sinusoidal as shown in FIG. 6b orsimilar undulated form in the radial direction. A significant level ofundulation is required to obtain the locking effect, preferably anundulation amplitude of 4-8 mm with at least one cycle of undulationwithin the dimension h of the tread. Since, in this embodiment, thesacrificial rib expected to undergo larger shear deformations than forthe previous embodiment, the narrow cut 40 may experience cracking ofthe rubber at the bottom of cut 40 due to both localized stressconcentrations and to increased shear deformation. To protect the tirefrom this adverse situation, the cut 40 is formed with an enlargedsection at the bottom of approximately two to five times larger than thewidth w₁. Whereas it is preferred in this embodiment for the depressiond to be initially zero, non-zero initial values as disclosed for thefirst embodiment are also acceptable. Thus the ratio R₁ will now be0≲R₁≲0.20:1 which for h=20 mm specifies 0≲d≲4 mm with d preferably equalto zero on the new tire.

FIG. 7a shows another example of the third embodiment of the inventionwhere the second cut 50 has a continuously variable width from treadsurface to the bottom of cut 50. In this case w₂ is measured at the mostradially outward location. The shape shown in FIG. 7a is curvilinear, sothat at approximately half tread depth, the width of cut 50 issubstantially equal to the width of the narrow cut 40. In this mannerthe buttressing effect of the sacrificial bridge is restored, but thedepression d and a favorable value of R₁ are maintained during the lifeof the tire. An alternative form for cut 50 is shown in FIG. 7b wherethe cut follows a substantially straight line taper from the treadsurface to a point radially inward and located at approximately halftread depth. From this transition point to the bottom 51, the width issubstantially constant and is preferably equivalent to that specifiedfor the narrow cut 40. In both examples shown in FIGS. 7a and 7 b, anenlargement 51 at the base of the cut is preferred to prevent stresscrack formation. Further resistance to the formation of surfaceanomalies can be obtained by inclination of either or both first cut 40and second cut 50 as described herein for the second embodiment of theinvention. In this instance the dispositions of inclined cuts shown inFIG. 4a and FIG. 4b also applies to the third embodiment.

What is claimed:
 1. A tire having a tread portion comprising: (a) aplurality of axially spaced apart essentially circumferential grooveshaving a depth h in the tread portion of the tire, and (b) at least onerib formed by the land portion between two of said circumferentialgrooves, and (c) a plurality of narrow transverse grooves having a depthh₁ not exceeding the depth h of said circumferential grooves andarranged at circumferential intervals on at least one of said ribs,wherein alternating pairs of said transverse grooves define first landportions having a circumferential length L₁ circumferentially adjacentto second land portions having a circumferential length L₂, the ratio ofthe length L₂ of said second land portion to the length L₁ of said firstland portion is such that 0.25:1≲L₂/L₁≲0.50:1, and (d) a first of saidpair of narrow transverse grooves disposed at a leading edge of saidsecond land portion having a width w₁ and a depth h₁, and a second ofsaid pair of narrow transverse grooves disposed at a trailing edge ofsaid second land portion having a width w₂ and a depth h₂, wherein thewidth w₁ of said first narrow groove is about 0.2 mm to about 1 mm andthe width w₂ of said second narrow groove is about 3 mm to about 10 mm.2. The tire according to claim 1, wherein a said depths h₁ and h₂ aresubstantially equal.
 3. The tire according to claim 1, wherein the widthw₁ of said first narrow groove is about 0.5 mm.
 4. The tire according toclaim 1, wherein said first narrow groove has an undulatedcross-sectional form in the tire circumferential direction, saidundulated form having an amplitude of about 4 mm to about 8 mm andhaving at least one cycle of undulation.
 5. The tire according to claim1, wherein said second narrow grove has a continuously variable groovewidth, said width having a maximum value at a radial location coincidentwith the tread surface, and a minimum value at a radially inwarddistance approximately equal to said depth h₂ of said second narrowgroove.
 6. The tire according to claim 1, wherein said second narrowgroove width is comprised of a first variable width section, said firstsection extending radially inward from said tread surface a distanceapproximately half said second narrow groove depth and having a maximumwidth at said tread surface and a minimum width at the radiallyinwardmost extent of said first section, and a second section ofconstant width equal to said minimum width and extending from theradially inwardmost extent of said first section to the radially inwardextent of said second narrow groove.
 7. The tire according to claim 1,wherein said first narrow groove has an angle of inclination relative tothe radially outward direction of about −5° to about −15°.
 8. The tireaccording to claim 1, wherein said second narrow has an angle ofinclination relative to the radially outward direction of about −5° toabout −15°.
 9. The tire according to claim 1, wherein said first narrowgroove has an angle of inclination relative to the radially outwarddirection of about −5° to about −15°, and said second narrow groove hasan angle of inclination relative to the radially outward direction ofabout −5° to about −15°.
 10. The tire according to claim 1 wherein bothfirst and second narrow grooves have an angle of inclination relative tothe radially outward direction of about −7° to about 10°.